Washington: At least seven companies from China, India, South Korea and South Africa continued to have investments in Iran's oil and gas sectors in 2012 even as Tehran came under international scrutiny for its nuclear ambitions, a US government watchdog said on Friday.

A new US law signed in August gave the Obama administration the authority to sanction firms that help Iran develop its energy resources, a key source of revenue for the country.

For a report to Congress required under that August law, the US Government Accountability Office reviewed publicly available reports and data, and contacted the companies involved to check whether they have stepped back from Iran.

"We did not attempt to determine whether the activities for which the firms are listed in our reports met the legal criteria for sanctionable activities," the GAO said, noting the State Department is responsible for those types of determinations.

The State Department reviewed the report ahead of its publication but declined comment, the GAO said.

The United States and European Union have targeted Iran's energy sector for sanctions, an effort to pressure Iran to stop pursuing nuclear weapons. Iran has said its nuclear program is for peaceful purposes.

The United States requires buyers of Iranian oil to make significant cuts to their oil purchases, or risk being cut off from the US financial system.

Most of the companies still involved in Iran's energy sector are from countries that on Friday received six-month waivers called "exceptions" to the sanctions because they have reduced oil trade.

Chinese activity included Sinopec's 51 percent stake in Iran's Yadavaran oil field, and China National Petroleum Corp's interest in a project to develop the Azadegan field, the GAO said.

South Korea's Daelim Industrial Co is helping develop the South Pars gas fields and is also involved in a liquefied natural gas project at Tombak, the report said.

Three Indian companies with stakes in the Farsi Block gas field -- Indian Oil Corp Ltd, ONGC Videsh Ltd and Oil India Ltd -- told the GAO that their exploration service contracts had expired and they had no plans to pursue further work on the project.

South Africa's Sasol has been active in a joint venture in Iran but recently stated it is trying to divest, the GAO said.

There were another eight companies from China, Malaysia, India, Croatia and Venezuela whose recent involvement in Iran was difficult to confirm, the GAO said. The companies did not all respond to the GAO's questions.

The GAO also examined whether foreign firms were selling refined petroleum products to Iran -- another activity now sanctionable under US laws.

Syria's Sytrol has been reported to have sold gasoline to Iran, and was sanctioned by the United States on Aug. 10.

The GAO found 12 of 17 firms had stopped selling petroleum products to Iran. The watchdog said it could not confirm whether an additional four firms from China and Venezuela were still active in the market.